Biology (Holt)

Harte’s data indicated that the ponds became more acidic when the
snow melted. After a few weeks, the pH rose and then leveled off. The
data confirmed Harte’s first prediction and supported his first hypoth-
esis—melting snow caused acid to be released into the ponds at
snowmelt, as shown in Figure 16. After snowmelt, the acid was neu-
tralized, probably by minerals that dissolved from the rocks in the
ponds, and pond pH returned to normal for the rest of the summer.
To confirm his second hypothesis (melting snow was making the
ponds acidic and harming the salamander embryos), Harte did an
—a planned procedure to test a hypothesis. Salamanders
lay eggs in the ponds once a year, as soon as pond ice melts. Harte
wanted to test whether exposure to the pH levels he had recorded at
that time of year would harm the salamanders that hatched from
the eggs.
Harte performed a controlled experiment. In a controlled experi-
ment, an experimental group (a group that receives some type of
experimental treatment) is compared with a control group. A
is a group in an experiment that receives no experi-
mental treatment. The control and experimental groups are
designed to be identical except for one factor, or variable. The factor
that is changed in an experiment is called the.
In Harte’s experiment, the independent variable was the acid (pH)
level. The variable that is measured in an experiment is called the
.Harte’s dependent variable was the number of
salamanders that hatched from the eggs.

dependent variable

independent variable

control group

experiment

SECTION 3Scientific Processes 17

Reviewing Information

On a separate sheet of

paper, make a table with

two columns. List the stages

common to scientific inves-

tigations in the left-hand

column. In the right-hand

column, describe in your

own words what is actually

done at that stage and

why that is important to

scientific inquiry.

Determining the pH of

Common Substances

You can use pH indicator paper to determine the pH

of various solutions. The pH indicator paper changes

color when it is exposed to a solution. The change in

color indicates how acidic or basic the solution is.

Procedure

1.Make a data table with three

columns and the following

headings: Solution, Predicted

pH, andMeasured pH. Make

a row for each solution to be

tested.

2.Predict the pH (acid or base)

of each solution, and record

your predictions in your data

table.

3. Testeach solution with pH
   paper, and record the results
   in the appropriate row in your
   data table.
   Analysis


4. Summarize your findings in
   two sentences.


5. Determine whether the
   predictions that you made

were correct. Explain any

differences between your

predictions and your results.

3. Compare your results with
   those of the rest of the class.
   Explain any differences.


4. List the steps of scientific
   methods that you followed in
   doing this activity.

Materials

paper, pencil, wide-range pH paper, three different

solutions, beaker or small jar, water

2B 2C